Iranian MPs held talks Saturday in Syria with President Bashar al-Assad and denounced any possible Western attack on that country over chemical weapons allegations, the head of the delegation said.
Meanwhile, the head of Iran's powerful Revolutionary Guards warned that a US strike would trigger "reactions beyond" Syria and would bolster extremism.
"As representatives of the Islamic Republic of Iran we declare our hostility for any attack or war against Syria," IRNA news agency quoted Allaeddine Boroujerdi, chairman of parliament's influential foreign policy committee, as saying.
US President Barack Obama on Friday said he was weighing "limited, narrow" action against Syria after US intelligence reports said President Bashar al-Assad's regime had gassed to death hundreds of women and children.
Boroujerdi said Iran informed the United States that rebels fighting to topple Assad have chemical weapons.
"Unfortunately the United States did not heed our warning," he said after heading a three-member delegation that met Assad.
Diplomatic relations between Iran and the United States have been severed since 1980, and the Swiss embassy in Tehran handles US interests there.
Syria's government has denied using chemical weapons and has pointed the finger of blame at "terrorists" -- its term for rebels seeking to topple Assad.
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Boroujerdi said Iran condemns the use of chemical weapons "by terrorist groups against the Syrian population."
Iran, Damascus's main regional ally, has issued stern warnings against any US-led military action targeting Syria, with the latest on Saturday by the head of the elite Revolutionary Guards.
Commander Mohammad Ali Jafari warned that a US strike would trigger "reactions beyond" Syria and would bolster extremism.
"The fact that the Americans believe that military intervention will be limited to within Syrian borders is an illusion; it will provoke reactions beyond that country," Jafari was quoted as saying by ISNA news agency.
"Just as US interventions in the Islamic world (Afghanistan, Iraq) have bolstered extremism, so will an aggression on Syria reinforce extremism and, as in Iraq and Afghanistan, its results will be pain, massacre and the exodus of the innocent population," he added.
Supreme leader Ayatollah Ali Khamenei has said a strike on Syria would be a "disaster" for the region.
And on Friday, Foreign Minister Mohammad Javad Zarif spoke by telephone with UN special envoy for Syria Lakhdar Brahimi to "warn against any adventurism that will have serious consequences".
Iranian media reported that Zarif also called a dozen European and Arab foreign ministers to "condemn the use of chemical weapons" in Syria, to "criticise the war-mongering (of the US and its allies) and raise concern about the consequences of any military action in the region".